The present invention relates to gas sensors mounted in the exhaust stream of an engine and more particularly to monitoring the deterioration of the sensors.
Oxygen sensors are employed in various applications to determine the oxygen content for a stream of gas as it flows past the sensor. One example of such an application is the use of an oxygen sensor mounted in the exhaust stream from an internal combustion engine. By monitoring the exhaust stream, one can determine various characteristics of the combustion events within the engine.
The information gained from the oxygen sensor can then be used, for example, as feedback control of the air/fuel (A/F) ratio. The A/F control allows for engine operation to be maintained around stoichiometry where emissions are most effectively reduced with a typical catalytic converter.
One example of such an oxygen sensor is a zirconium-oxide (ZrO2) oxygen sensor. This sensor is made up of a yttrium doped ZrO2 ceramic, with two sides of the ceramic covered with a respective one of two electrodes, usually platinum, to form an electrochemical cell. A porous layer may also be located on one or both of the electrodes. The performance of an oxygen sensor comprises both static and transient characteristics. The static characteristics of a zirconia oxygen sensor are defined by the magnitudes of the voltages for rich and lean readings and the switching point of the sensor. The transient characteristics of a zirconia oxygen sensor are defined by the response times.
The response times of the sensor define the transient characteristics of an oxygen sensor. The response times are a measurement of the times that an oxygen sensor needs to switch from high voltage (rich state of the exhaust) to a low voltage (lean state of the exhaust), or vice versa. The effects of varying response times are manifested in the limit-cycle frequency of the A/F modulation. An oxygen sensor with shorter response times generally results in an A/F ratio that remains closer to the desired value.
As a sensor ages, however, build up of contaminates may occur on the sensor, thus causing a deterioration in the response time, and thus the effectiveness of the sensor. How much deterioration must occur before a sensor becomes unacceptable is difficult to quantify. Thus, it is desirable to have a way to detect and quantify the deterioration of an oxygen sensor as it ages, in order to determine when it becomes unacceptable for its intended purpose.
In its embodiments, the present invention contemplates a method of monitoring the deterioration of a sensor in an exhaust stream of an engine. The method includes the steps of: operating the engine under a condition where the oscillation frequency of air/fuel ratio between rich and lean is determined; receiving a signal from the sensor; performing a Fourier Transform on the signal to produce a frequency domain signal having even and odd order harmonics; calculating the magnitude of one of the odd order harmonics and one of the even order harmonics; calculating a ratio of the odd order harmonic to the even order harmonic; and comparing the ratio to an acceptable range of values.
Accordingly, an object of the present invention is to monitor the deterioration of an oxygen sensor as it ages by employing a Power Density Analysis of a Fourier Transform to the output signal from the sensor.
An advantage of the present invention is that the deterioration of one or more oxygen sensors can be monitored by a relatively simple process with a high degree of accuracy.
Another advantage of the present invention is that it can be employed while the engine operates under a fixed (forced) air/fuel modulation frequency, or, if desired, under a closed-loop limit-cycle control with proper baseline calculation.
A further advantage of the present invention is that multiple sets of different order harmonic ratios can be employed to assure that a false failure output is not generated.